1. Find silage dry-matter demand
Animal count, body weight, total dry-matter intake and this silage's ration share determine the dry matter the herd consumes each day.
Silage Storage and Bunker Sizing Calculator
Size rectangular bunker storage from herd dry-matter demand, or check the capacity and feedout rate of an existing bunker.
How the Silage Bunker Calculation Works
2. Allow for separate losses
The consumed amount is divided first by the retained fraction after feeding loss and then by the retained fraction after storage loss.
3. Convert mass to bunker volume
Dry matter percentage converts packed DM to as-fed mass. Packed as-fed density converts mass to cubic storage volume.
4. Size the feedout face
Target daily face advance and feeding days establish total feedout length. Volume divided by that length gives the active face area.
Formulas and Assumptions
Daily silage DM consumed = animals × body weight × DMI fraction × silage share of ration DM
Daily silage DM removed = consumed DM ÷ (1 − feeding-loss fraction)
Packed DM required = daily removed DM × feeding days ÷ (1 − storage-loss fraction)
As-fed silage required = packed DM ÷ silage-DM fraction
Storage volume = as-fed mass ÷ packed as-fed density
Sizing mode: face area = volume ÷ (face advance × feeding days)
Sizing mode: average width = face area ÷ average packed depth
Existing bunker: capacity = average width × average packed depth × usable length × bunker count
Dimensions are internal and the bunker is treated as a rectangular prism. Sequential bunkers are assumed to have the same cross-section and to be opened one at a time. Density is assumed uniform. The calculation does not model settlement, side slopes, a crowned surface, end wedges, wall batter, spoiled caps, residual inventory or multiple simultaneously active faces.
Worked Example: 200 Animals and Two Bunkers
With 200 animals averaging 1,400 lb, total DMI at 3% of body weight, and this silage providing 50% of ration DM, daily silage consumption is 4,200 lb DM. At 8% feeding loss, 15% storage loss and 35% silage DM, about 2,801 U.S. tons as-fed must be placed for 365 feeding days.
At 45 lb as-fed/ft³ and a 12 in/day target face advance, required volume is about 124,500 ft³ and active face area is about 341 ft². With 12 ft average packed depth, average internal width is about 28.4 ft. Two sequential bunkers would each need about 182.5 ft of usable length.
Calculation Basis and References
- University of Wisconsin Extension: Bunker Silo Sizing documentation — herd silage DM need, separate storage and feeding losses, packed density, moisture, face removal, storage period and practical bunker dimensions.
- University of Wisconsin Extension: Feedout losses from forage storage systems — as-fed density and face-removal guidance, including a 12 in/day design value.
- University of Wisconsin Extension: Packing bunker and pile silos to minimize porosity — minimum density discussion and the relationship between density, porosity and loss.
- Penn State Extension: Horizontal silo safety — bunker, pile and feedout hazards and safe-management considerations.
References support the planning method and warning thresholds. They do not certify any particular ration, wall, floor, drainage system, equipment plan or silage face.
Silage Storage and Bunker Sizing FAQs
How is required silage storage volume calculated?
The calculator finds daily silage DM consumed, adjusts sequentially for feeding and storage losses, converts DM to as-fed mass with silage dry matter percentage, and divides the as-fed mass by packed density.
What packed silage density should I use?
Use a measured density from the same forage and storage whenever possible. The 45 lb as-fed/ft³ default is a planning value, not a guaranteed result. Crop DM, chop, packing weight, layer thickness, delivery rate and operator practice can move actual density substantially.
What face removal rate should I design for?
University of Wisconsin Extension recommends 12 in/day as a design value. Its guidance says removal should not fall below 4 in/day in summer or 3 in/day in winter, and rates above 6 in/day better limit feedout loss. Warm conditions, low density and an uneven face can require faster progress.
Why does the tool ask for average internal width?
The rectangular capacity formula needs a representative width. Measure between the inner wall faces near mid-height when the walls are nearly vertical. Use a more detailed survey for sloped, irregular or drive-over piles.
Are storage and feeding losses added together?
No. They are applied sequentially. For example, 15% storage loss and 8% feeding loss retain 0.85 × 0.92 = 78.2% of packed DM, not 77%.
Can this calculator approve bunker walls or face height?
No. It does not calculate lateral wall loads, foundations, drainage, equipment turning space or safe face height. Use qualified agricultural engineering and local safety guidance for construction and operation.
Are my herd and feed values stored?
No. All calculator logic runs client-side, and Starlight Tools does not send or store the values you enter.
Planning and Safety Limits
This calculator is for preliminary feed inventory and geometry estimates. Confirm ration inputs with a qualified nutrition adviser and verify crop dry matter and density with representative measurements. It does not design retaining walls, slabs, drainage, runoff controls, covers or traffic routes. Silage faces can collapse without warning; never work beneath an overhang, keep people and equipment outside the fall zone, and ensure feedout equipment can safely reach the top without undercutting. Follow applicable farm-safety rules and obtain site-specific engineering for construction or alteration.